This invention relates to protective relaying. More specifically, this invention relates to high speed, intelligent protective relays for use in protecting power transmission systems.
Alternating Current ("AC") power distribution systems are well known and have been in conventional use for decades. An AC power system functions to generate power and distribute it to consumers through high voltage transmission lines.
Typically, such AC power systems operate at normal, steady-state conditions with voltages and currents at or near their rated (i.e., acceptable tolerance) values. On occasion however, "fault" (e.g., short circuit) and other abnormal conditions do arise. During such conditions, the voltages in the faulted area, for example, may drop far below nominal values and the relevant line currents may rise to very high values.
In an effort to address this problem, AC power distribution systems conventionally include protective relays installed about various equipment components and elements of the power system. Such relays are capable of detecting and isolating fault conditions in defined areas of the system, and are also capable of disconnecting the area or equipment subject to fault. Protective relays conventionally accomplish this task via control of properly located circuit breakers.
Thus, the protective relays function to remove from service a faulty portion of the system while maintaining service continuity in the healthy part of the system. An additional benefit of protective relays is that they can isolate and provide an indication of the location of the fault. Thus, such relays can aid in future resolution of the fault causing condition.
Protective relays come in a variety of types. Distance relays, which use both current and voltage inputs and output a trip condition, are very popular devices for transmission line protection. Distance relays make trip decisions based on whether there is a fault in a pre-defined zone extending from the relay location. They can be implemented using systems which determine an apparent fault distance and explicitly compare it with a pre-defined distance setting, or by comparing two sinusoidal signals, implicitly making the same determination.
Current (or voltage) relays make trip decisions by comparing an incoming current (voltage) input with a pre-defined current (voltage) setting. In the case of over-current relays, the relay generally makes a trip decision based on whether current exceeds the pre-defined current tolerance. In the case of under-voltage relays, the relay generally makes a trip decision based on whether voltage is lower than a pre-defined voltage constant.
Despite the benefits of protective relays, many existing protective relays have performance characteristics which are sub-optimal for a variety of applications. For example, for distance relays, in order to find the apparent distance (or impedance) of the fault, the signal processors may conventionally perform a Discrete Fourier Transform (DFT) algorithm on current and voltage inputs to determine the current and voltage phasors. Performing the DFT transform, which usually involves signal sampling at numerous data points and the performance of multiple operations, can be slow. Thus, distance relays implementing the DFT algorithm may be unsuitable for ultra-high speed applications.
Another shortcoming of some conventional protective relays is that parameters affecting current and voltage fault values are not taken into account. As a result, a relay may make inaccurate trip decisions. For example, due to the energy storing elements in a power system, fault voltages and currents contain DC ("Direct Current") offset. DC offset is an exponentially decaying DC quantity which depends on the time constant of transmission lines. Although DC offset will tend to affect input parameters processed by conventional protective relays, relay signal processing may not take DC offset into account. Thus, these inaccurate inputs may cause conventional relays to overreach and lose security (i.e., initiate a trip decision sooner than is desirable).
A related problem with some conventional protective relays is that they exhibit limited selectivity properties. For example, conventional relay signal processing may not distinguish between faults well within the relay's zone of protection, on the one hand, and faults on or near the zone border on the other. Such relays may not be flexible enough to provide both operating speed when the fault is well within the zone reach and security when the fault is near the balance point.